limiting CCD magnitude for SN searching = f(???)

[rmuhlack]

Is there a method for calculating a theoretical limiting magnitude as a function of integration time, aperture, f-ratio, and sensor QE (and any other relevant parameters)?

I would like to be able to calculate this for both point and diffuse sources.

[Amaranthus]

I suspect sky background would also be important (overriding?).

[rmuhlack]

Good point Barry. I agree that sky background (light pollution, moon etc) would also factor in.


My original question is in the context of supernova searching. Given a fixed scope/ccd combo, how would I determine the limiting magnitude for say a 1min exposure? I have changed the thread title now to reflect this

[Amaranthus]

I agree a formula would be useful, but I don't know of one. A mag zero star produces ~1 million photons/cm2/s in the visual-band. So that's 10,000 for mag 5, 100 for mag 10, 1 for mag 15 and 0.1 photons/cm2/s for mag 20. Would that be detectable by your camera chip vs sky signal?

You could do it by trial-and-error, perhaps? You'd need a chart that goes deep (mag 16+) and could then cross-check your image (with different exposures) against known-magnitude stars.

[Amaranthus]

This may be useful Richard: http://www.astro.caltech.edu/~george/ay20/Ay20-Lec3x.pdf

[Shiraz]

seeing comes into it as well for point sources.

[rmuhlack]

A field trial (say over a full lunar cycle) would certainly clarify things for my imaging location, however at this stage its still a theoretical consideration as I dont currently have all the equipment (in particularly the CCD).

I guess its my process engineering background coming through here - build a (mathematical) process model where possible and the use it to optimise prior to actually committing capital.

I already have a 200mm f4 newtonian, and have been contemplating pairing this with an ICX674 based camera (eg Atik 428EX mono). This would give a peak QE of 77% and an image scale of 1.17 arcsec per pixel. The short focal length and light-weight payload would be well within the capability of a NEQ6. What I would like to be able to do is work out the limits of this equipment combo to evaluate whether it is a project that is worth pursuing.

[rmuhlack]

i thought so. for this reason I was (perhaps incorrectly) thinking that I would not be at a major disadvantage with a 200mm f4 scope and an image scale of 1.17

[SamD]

Short answer, for 1 min unfiltered exposures, about 17.9 with a 200mm f/4 and at 1.17 arcsec/px (assuming you have pretty dark Mag 20 / sq arcsec skies).

With my light pollution, I need about 5 mins to get the same result with similar equipment (see the png). Star mags on the image are from GSC2.3, which goes to about mag 20. Once you get near about mag 18 in my image, you have to use a bit of imagination to see the stars above the sky glow noise.

Attached my excel calculator, which seems to more or less agree with what I find when looking at my images. The spreadsheet also predicts what the background sky glow should be which you can check against your images (you have to convert the sensor electrons to your camera ADU units)